It's a very good question and I needed to contact iBasso to check, so apologies for the delay responding.

The rechargeable battery in the iBasso D2 is indeed replaceable. According to iBasso the replacement procedure is relatively simple.

Battery replacement is a service iBasso themselves offer. iBasso have also offered us the possibility of taking on spares for their amps and we are investigating the possibility of servicing the battery ourselves.